The present invention relates to switching regulators. Switching regulators have long been used for providing a regulated voltage. Various power supply designs incorporating switching regulators have been developed. Various attempts at achieving fast transient response to transients in the input voltage or in the load have been made. A discussion of the art of switching regulators can be found in U.S. Pat. No. 5,770,940 (Goder). The full disclosure of the Goder patent is hereby incorporated herein by reference. The Goder patent itself provides one of the most recent designs for providing fast transient response. The Goder circuit includes a fast feedback loop and a slow feedback loop to provide a highly responsive switching stable switching regulator.
Power supplies are now being used in great quantities for microprocessors and other computer equipment. This equipment is placing greater and greater demands on power supplies. A load can rise instantly from a minimum to a maximum level when a system wakes up from a power down mode. The load current can also experience an abrupt drop when the system reduces the power consumption. To keep the output voltage tightly regulated during this transition has been a difficult challenge. Another transient condition which modern computer technology is beginning to impose on power supplies is the ability to respond when the targeted voltage changes. For example, output computers will be designed in which the supply voltage will vary depending upon the supply source, such as between batteries or power line. The computer will require that the power supply promptly re-regulate the supply voltage when the supply source is switched giving a new target voltage.
The existing switching regulators use an error amplifier to achieve output regulation. Feedback voltage from the output is compared with a reference voltage to generate an error signal. The error signal is then digitized through use of a pulse width modulator comparator to modulate the switching devices in a power supply. The on and off ratio of the switching devices then controls the power flow for voltage regulation. Since the error amplifier has limited bandwidth, the error signal inevitably has a delay relative to a deviation in the output voltage, or in the reference voltage. Increasing the bandwidth on the error amplifier tends to cause ringing on the error signal and thus on the output voltage. Such a system would be unstable.
In accordance with embodiments of the present invention, the output of an error amplifier is summed with a target signal. Examples of target signals include a reference voltage or a targeted load current. The sum is fed into the comparator for comparison with a ramp signal. In an embodiment of the invention, the ramp signal is derived from the ripple on the output voltage. The comparator produces a control signal to control the switching of the switching regulator.
In accordance with an embodiment of the invention adapted for the dual feedback switching regulator control described by Goder, an error amplifier receives a signal proportional to an output voltage of the switching regulator and has a reference input connected to a modifiable reference voltage. The reference voltage may be changed, for instance, according to the specifications for a new notebook computer, if a supply is changed from a battery to a power line. The error signal from the error amplifier and a signal proportional to the modifiable reference voltage are summed. The control signal is produced by a comparator having an input coupled to the summer and an input connected to receive a ramp signal derived from the output voltage of the switching regulator.
In a current mode control embodiment of the invention, the error amplifier receives a signal proportional to the output voltage of the switching regulator and has a reference voltage input. The switching regulator provides current through an inductor. A current sensor provides a current sense signal proportional to the load current of the switching regulator. The load current sense signal is summed with the output of the error amplifier. A comparator having an input coupled to the summer and an input connected to receive a ramp signal derived from the inductor current of the switching regulator produces a control signal for the switching regulator.
A method of the invention for fast and slow feedback control includes generating a periodic ON and OFF control signal. The signal proportional to the output voltage of the switching regulator is compared with a modifiable reference voltage to develop an error signal. The error signal and the signal proportional to the modifiable reference voltage are summed to produce a voltage sum signal. The voltage sum signal is compared with a signal derived from the output voltage to adjust the duty cycle of the periodic ON and OFF control signal. The ON and OFF control signal causes the switch to open in one state and close in response to the other state.
Embodiments of the invention advantageously respond quickly to changes in the targeted voltage or current. Other objects and advantages of the invention shall become apparent during the following description of the presently preferred embodiment of the invention taken in conjunction with the drawings.
FIG. 1 is a schematic diagram of a switching regulator controlled by an embodiment of the invention.
FIG. 2 is a schematic circuit diagram of a switching regulator controlled by an alternate embodiment of the invention.
FIG. 3 is a schematic circuit diagram of an alternative switching regulator controlled by an embodiment of the invention.
FIG. 4 is a schematic circuit diagram of a switching regulator controlled by current mode control in accordance with an alternate embodiment of the invention.